Our Floe lift uses 4" white PVC pipe (vertically) for guides, which for the first 10-15 trips of the year get used a lot! I think another design that helps keep scuff marks to nothingness is having them oversized so they can actually roll with the boat sliding across them. The aluminum part of the guide is made of a 1.5x3" tubing so there's lots of room for it to roll around.